The present invention generally relates to air separation membranes and, more particularly, to photo-crosslinked hollow fiber membranes for use in air separation modules (ASMs).
ASMs are the key component for the nitrogen generation systems (NGS) needed to provide fuel tank inerting for commercial and military aircraft. Useful membranes for separating oxygen from nitrogen must have sufficient selectivity to distinguish between these two similar gases, and must also have high flux. Since flux determines the size and weight of the ASM needed for a given product flow, maximizing flux is key to fitting the ASM into the aircraft.
Flux is generally quantified as either permeance or permeability. Permeance, measured in Gas Permeation Units (GPU), is the flow of gas through the membrane at standard temperature and pressure (STP), divided by the membrane area and the trans-membrane pressure drop: 1GPU=10−6 cm3 (STP)/cm2 S (cm Hg). Permeability, measured in Barrer, is the flow, multiplied by the membrane thickness and divided by membrane area and pressure drop: 1 Barrer=10−10 cm3 (STP) cm/cm2 s (cm Hg). Flow through the membrane will increase with increasing membrane area or trans-membrane pressure drop, and will decrease with increased membrane thickness.
Membranes for use in the fuel tank inerting systems of commercial aircraft have special requirements. The air used as the feed to the membrane system will frequently come from the engines as a “bleed air” stream. The temperature of this stream may be higher than 300° C. It is therefore cooled before introduction into the membrane. The size of the cooling system and the volume of coolant should be minimized to reduce weight in the aircraft, so the membrane system will be operated at as high a temperature as possible, generally at least 140° F., and more commonly 200-250° F. Even higher temperatures are desired. The inlet air will commonly contain ozone since the ozone concentration in air increases with altitude, and may contain hydrocarbons from either the environment around the aircraft or from the engine itself. The membrane must tolerate these contaminants. Finally, the membrane module must have a long useful life without requiring maintenance, and should be very reliable. These requirements rule out many common materials of construction for air separation membranes, and make many commercially available membranes unsuitable for the aircraft application.
Obviously, to make membranes which have as high flux as possible, one wishes to make the membrane very thin. Since a thin membrane would also be very fragile, most membranes are anisotropic, and have a very thin selective layer, supported on a porous support. There are two basic types of anisotropic membranes, the asymmetric, or Loeb-Sourirajan, membrane in which the selective layer and the support have the same chemical structure, and the thin film composite membrane, in which they are different.
U.S. Patent Application No. 2006/0011063 discloses a gas separation membrane formed from polyetherimide by extruding a hollow fiber using a core liquid. For the described membrane, like other asymmetric hollow fiber membranes, one polymer solution is spun from an annular spinneret and the core liquid is pumped into the center of the annulus. Generally, the nascent hollow fiber membrane passes through an air gap into a nonsolvent coagulation bath, followed by wind-up on a drum, roll or other suitable device. The fiber velocity is accelerated in the air gap from the extrusion velocity at the spinneret exit to the wind-up velocity of the drum. The wind-up velocity is usually adjusted to elongate the fiber and to draw down the diameter of the nascent fiber in the air gap to the desired finished fiber diameter. Generally, increasing the wind-up velocity increases fiber surface area and reduces fiber outer diameter (OD). “Draw ratio” is a commonly used parameter for characterizing the degree of extensional deformation that the fiber experiences in the air gap and is the ratio of wind-up velocity to the average extrusion velocity. The coagulant fluid leaches solvent from the annular stream to form a denser region near the outside surface of the fiber that becomes the selective layer of the fiber, while the bulk of the fiber becomes porous. Since only one polymer solution is used for asymmetric membranes, the polymer must fulfill all of the requirements for the fiber, including low cost, high permeance, high selectivity, mechanical strength etc.
U.S. Pat. No. 6,805,730 discloses porous hollow fiber membranes having convoluted inside and/or outside surfaces. The convoluted surfaces increase flux by increasing the surface area of the fiber. The preferred membranes are described as integral, i.e., they do not have a plurality of layers laminated together. In a more preferred embodiment, the integral membrane is all of one composition. Although the described fibers may provide increased surface area and increased flow through the fiber, the described membranes are asymmetric membranes wherein one polymer solution fulfills all the requirements of the fiber.
In contrast to asymmetric membranes, thin film composite membranes include more than one polymer solution. For thin film composite membranes, one polymer is used to create the porous bulk of the fiber (core), while a second polymer is coated on the surface and becomes the selective layer (sheath). Now the bulk polymer can form a mechanically strong porous fiber, while the thin film polymer can have high permeance and selectivity. Separating these requirements using thin film composite membranes allows many more polymers to be used than in the asymmetric approach. The thin film polymer may be applied after formation of the porous membrane fiber by techniques such as dipping, casting, or interfacial polymerization, or it may be applied by co-extrusion during spinning of the fiber. In other words, thin film composite hollow fibers may be made by coating a polymeric film onto an already made core membrane fiber, by doing a polymerization on the surface of an already made core fiber, or by co-extruding the core and sheath layers comprising different polymers simultaneously into the water bath.
U.S. Pat. No. 6,663,805 discloses a process for making hollow fiber mixed matrix membranes. The '805 patent describes both asymmetric and composite membranes. The mixed matrix membranes are characterized by a continuous phase of selectively gas permeable polymer in which are uniformly dispersed discrete absorbent particles such as molecular sieves that also have selectivity enhancing properties. The fibers of the '805 patent can be monolithic in which the fiber wall is entirely mixed matrix, or composite in which an active mixed matrix layer is positioned adjacent to a supporting substrate layer. It was noted that permeance increased gradually up to draw ratio of 6.2 and increased at slightly higher rate for draw ratio of 7.4. Fiber deformation resulting from draw down is said to be beneficial because it provides increased surface area for permeation per unit volume of the hollow fiber module. Although the absorbent particles may enhance fiber selectivity, they add complexity and cost to membrane production. Although increasing draw ratio may increase permeance, still further increases are needed for some applications.
As can be seen, there is a need for improved air separation membranes. Membranes are needed wherein the core layer and the sheath layer can be optimized separately. A low cost separation membrane having high permeance, high selectivity and operability at high temperatures is needed.